U.S. Pat. No. 3,773,740, issued Nov. 20, 1973 in the name of T. T. Szabo, assigned to Union Carbide disclosed the devolatilization of polymers in a flash chamber. Typically the polymers are polymers containing at least one vinyl aromatic monomer. The patent teaches that from 0.50 to 2.75 weight % of water may be injected into a melt of the polymer. The pressure on the polymer melt is suddenly lowered to about 20 to 40 mm of Hg (torr). The water in the polymer melt is flashed to help remove residual vinyl aromatic monomers to about 0.30 weight % or about 3,000 parts per million (ppm).
U.S. Pat. No. 4,195,169, issued Mar. 25, 1980, assigned to The Dow Chemical Company discloses devolatilizing polymers of styrene and acrylic acid or methacrylic acid by contacting the polymer melt with a compound of the formula ROH wherein R may be hydrogen or an alkyl radical. The devolatilization process does not increase the gel content in the resulting polymer (i.e. there are no insolubles in the resulting polymer). The polymers of the present invention are of a different polymeric character in that they do not contain any free carboxylic acid.
Currently, the producers of polymers of vinyl aromatic monomers are seeking to produce polymers or blends of such polymers which contain less than about 200 ppm of monomers, oligomers and solvent.
The Union Carbide patent does not teach one how to reduce monomer, oligomer and solvent levels to those required today. A simple approach might be to merely further reduce the pressure within the devolatilizer. However, at pressures of about 10 torr and less, the water injected into the polymer melt cannot be cooled to sufficiently low temperatures to condense, because it will freeze at such low pressures in the condenser system between the devolatilizer and the vacuum source. Vapour pressure tables of water show at pressures of less than 4.579 mm of Hg water has to be cooled to less than 0.degree. C. to condense. Accordingly, if the pressure in the condenser is less than about 5 mm Hg it is very difficult to keep the system operational.
Operating under very closely controlled procedures and by selecting lots of polymer from a particular batch about the lowest levels of residual monomer, oligomer and solvent that can be obtained are in the range of greater than 175 ppm, typically from 200 to 175 ppm.
Applicants have discovered that one method for overcoming the drawback of the Union Carbide technology is to maintain the pressure within the devolatilizer lower than the pressure within the condenser. One method for achieving this pressure differential is to have a fluid ejector between the devolatilizer and the vacuum source. It is possible to eject a fluid such as steam at this point so as to maintain the very low vacuum required in the devolatilizer yet have a higher pressure in the condenser so that water won't freeze in the condenser.
As a result it is possible to operate a water assisted devolatilizer at pressures below about 10, preferably less than 8, most preferably less than 5 torr and to consistently obtain polymers having levels of monomers, dimers, trimers and solvents of less than 150, preferably less than 100 ppm.
U.S. Pat. No. 5,102,591, issued Apr. 7, 1992 discloses a process to devolatilize a polymer blend of styrene and polyphenylene oxide by passing the blend through an extrusion devolatilizer. That is an extruder equipped with vacuum ports. In such a process the polymer or polymer blend does not descend vertically through a flash chamber. Rather, the melt is passed horizontally in the barrel of an extruder beneath a vacuum port. Additionally, the reference teaches a two stage devolatilization. That is, first the polyphenylene oxide is devolatilized then the polystyrene is added to the polyphenylene oxide and the blend is then devolatilized.
U.S. Pat. No. 5,145,728 discloses reducing the residual monomer and oligomer content of polystyrene by blending with it a block copolymer of styrene and butadiene, typically such as those sold under the trade mark K RESIN. The reference does not contemplate passing the polymer melt through a flash chamber devolatilizer. Rather, the polymer is devolatilized conventionally, then extrusion blended with the block copolymer. In the example at columns 4 and 5, the starting polymer is devolatilized using a screw extruder and water. Interestingly, the residual monomer and solvent level was not reduced below 150 ppm. The present disclosure does not contemplate such a process and does not contemplate a blend comprising block copolymers of a block of one or more C.sub.8-12 vinyl aromatic monomers which are unsubstituted or substituted by a C.sub.1-4 alkyl radical and one or more blocks of one or more C.sub.4-6 conjugated diolefins.
The process of the present invention has an advantage over the extrusion processes as there is a shorter history of shear under high temperature. Each time a polymer blend is passed through an extruder there is some degradation of polymer, particularly a reduction of molecular weight. Furthermore, extrusion processes are relatively expensive.